With reference to FIGS. 1 and 2, which show traditional combustion chambers, premixed combustion chambers 1 comprise a plurality of mixing devices 2a, 2b all connected to a front plate 3 of a combustion device (thus all the mixing devices 2a, 2b have the same axial position with respect to a longitudinal axis of the combustion chamber 1).
Typically the mixing devices 2a, 2b are arranged in one, two or more rows around the combustion device and are connected to a fuel supply circuit in groups of three, four or five mixing devices, each group includes a plurality of mixing devices 2a and usually one or two mixing devices 2b. 
During operation, the mixing devices 2a are supplied with the nominal amount of fuel and, in order to counteract pulsations, the mixing devices 2b are supplied with a reduced amount of fuel, such that they are operated at a lower temperature; in other words the temperature of the flame generated by the mixture formed in the mixing devices 2b is lower than the temperature of the flame generated by the mixture formed in the mixing devices 2a. 
This structure limits the regulation possibilities, in particular at part load.
In this respect, FIG. 3 shows the relationship between power and flame temperature in a traditional gas turbine; Tp indicates the critical flame temperature below which large pulsations are generated within the combustion chamber.
From this figure it is clear that when operating at full power, the operating point 5 has a flame temperature Tf well above the flame temperature Tp, such that safe operation can be carried out.
Nevertheless, when the required power decreases (i.e. at part load), the operating point 5 moves along a line 7 towards the temperature Tp.
Since the flame temperature Tf must always be above the temperature Tp, a minimum power Pmin can be identified, such that safe operation at a lower power is not possible, because it would cause large pulsations that would inevitably damage the gas turbine.
It is clear that Pmin should be as low as possible, because in case only a very small power is needed (like in some cases during night operation of power plants) a substantial amount of the power produced is wasted; typically Pmin can be as high as 30% and in some cases 40% of the full power.
In order to increase the operating windows and safely operate the gas turbine at low power, combustion chambers are often provided with pilot stages.
Pilot stages consist of fuel injectors within the mixing devices; since pilot stages are only arranged to inject fuel (i.e. not a mixture of a fuel and oxidiser), they generate a diffusion flame that, on the one hand, helps to stabilize the combustion of the lean mixture generated at part load within the mixing devices, but on the other hand, causes high NOx emissions.
Alternatively, US 2010/0170254, which is incorporated by reference, discloses a combustion chamber with mixing devices supplying an air/fuel mixture into a combustion device (to generate a premixed flame). At the end of the combustion device, a second stage made of fuel and air injectors is provided; fuel and air are injected separately such that they generate a diffusion flame (i.e. not a premixed flame). Again, diffusion flames cause high NOx emissions.
U.S. Pat. No. 5,983,643, which is also incorporated by reference, discloses a combustion chamber with premixed fuel supply devices that are shifted along the combustion device longitudinal axis, but the flames generated by burning the mixture generated by all the mixing devices are downstream of all mixing devices.